Protective device for the electrical system of aircraft



April 17, 1962 J. LIKER 3,030,473

PROTECTIVE DEVICE FOR THE ELECTRICAL SYSTEM OF AIRCRAFT Filed July 1, 1959 In 1'11"" "Mun un) In ""1.

D C IOOO sac. r K

I l l l l I l l l m l g l- 0 CURRENT fig;

INVENTOR JACK. LIKEE ATTO R N EY 3,930,473 Patented Apr. 17, 1962 3,030,47 3 PROTECTIVE DEVICE FOR THE ELECTRICAL SYSTEM OF AIRCRAFT Jack Liker, East Norwallr, Conn., assignor to Burudy Corporation, a corporation of New York Filed July 1, 1959, Ser. No. 824,368 1 Claim. (til. filth-123) My invention relates to a protective, device"for the electrical system of aircraft. Such a protective device must necessarily possess the highest degree of reliability to prevent accidental loss of a vital circuit during flight. However, present protective devices have frequently been characterized by what has been termed nuisance tripping. When this happens, a vital circuit may be needlessly opened rendering it useless, for a critical period of time, when the circuit was not in fact over-loaded or only moderately so.

I have found that nuisance tripping occurs as a result of changes in the characteristics of the protective device brought about by changes in ambient temperature and/or a sensitivity to the accumulated effect of a number of small over-loads, none of which is critical. This objectionable feature is derived from the conception that it is desirable for the protective device to heat up and dissipate its heat at the same rate as the circuit component it is desired to protect.

Accordingly, the primary object of my invention is to provide a protective device having characteristics rela tively unaffected by changes in ambient temperatures, and which is not sensitive to the cumulative effect of a number of small or moderate overloads.

I found that certain metals used for these protective devices are sensitive to changes in ambient temperatures. These metals exhibit a large time-current characteristic tolerance band, i.e. they require large tolerance limits for purposes of design.

Accordingly, another object of my invention is to ascertain those metals which disclose a close tolerance, i.e. a narrow time-current characteristic band.

A further characteristic of those metals which are sensitive to the cumulative elfect of a number of small or moderate over-loads, is a change which takes place when the fuse is heated to a temperature in the vicinity of its melting point. These metals become plastic below the melting point and are not likely to retain the original accurate time-current characteristic needed to prevent a premature circuit opening in aircraft.

Another object therefore consists in providing fusible metals for protective device which are not plastic substantially below the melting point of the metal.

In the course of my experiment, I found that low melting point metals possess the objectionable large timecurrent tolerance bands. I, therefore, exclude such melting metals as lead and tin.

To make a high melting point metal effective to more quickly protect a circuit which may contain high melting point components, I have found it necessary to provide auxiliary heating means adjacent the protective element to make it operate efiectively and make its response to an overload more rapid. Thus, I place the heater in series with the protective element to build up the heat faster in the protective element than it does in the circuit. However, there is a tendency for the fusible element in the protective device to fuse at or near the junction between the heater and the fusible element. In order to maintain an accurate melting time-current characteristic, it is necessary to control themelting of the fusible element, to make it as close as possible to the center of the element.

Therefore, another object of my invention is to provide a means for insuring that the fusible portion of the protective device will fuse as close to the center as possible. a

I accomplish these and other objects and obtain my new results as will be apparent from the device described in the following specification,particularly pointed out in the claim, and illustrated in the accompanying drawings in which:

FIG. 1 is a side elevation, in section, of a protective device employing the invention;

FIG. 2 is' a time-current chart comparing the character istics of the present device with former devices.

Specifically in FIGURE 1 of the drawing, I have illustrated the protective device which is used in my system. It comprises two heater wires 10 and 12 joined as by welding to an intermediate fusibleelement 14. At the junction, the fusible element overlaps the heater wires, to form extensions 16 and 18. The wires and element may be supported in a conventional fire proof casing 20 having terminal contacts Hand 24 for supporting the wires 10 and 12.

In accordance with my invention, I employ a high melting point fusible element such as silver. .The heater wires may be made of nickel. I may also use tungsten in place of nickel and copper or aluminum in place of silver. It is essential that the heater element be of a higher resistance material than the fusible element to achieve the preferred functioning of my invention, wherein, the auxiliary heater Wires do not melt before element 14 fuses. Of course, it is possible to alter the time-current characteristics of the device by allowing the element 14 to fuse first at one end of the characteristic curve but reversing the fusing and permit the heater wire to fuse first at the other end of the characteristic curve.

In the chart of FIGURE 2, the time-current characteristics of the protective device in FIGURE 1, employing a high melting point fusible element such as silver, is shown by curve A. The fusible characteristics of such a metal will not change with changes in the ambient temperatures, nor by a series of over-loads of short duration.

On the other hand, if the fusible element is made of low melting point metal, the curve B represents the timecurrent characteristics after a single overload; curve C, the time-current characteristics after a second overload; and D, the same for a third overload. Accordingly, the high melting point metals are preferable because of their uniformity in the time-current characteristic band. The overlap sections 16 and 18, I have found prevent the junctions from fusing when the auxiliary heat reaches the fusible element. I believe that the extensions form heat dissipators which keep the junctions cooler than the center of the fusible element. The length of the extensions need be only /8 of an inch. No benefit accrues from using more than this length.

The protective system thus described is especially useful in aircraft, since nuisance tripping is eliminated. A fusible element of low melting point even when auxiliary heater elements are employed to simulate conditions in the components of the circuit, will not prevent the undesirable changes'in the time-current characteristics, which, in an aircraft, could result in the unwarranted opening of a vital circuit. My protective device is not responsive to changes in ambient temperatures and/or the accumulative effect of a numberof overloads each of noncritical magnitude.

I have thus described my invention, but we desire it understood that it is not confined to the particular forms or uses shown and described, the same being merely illustrative, and that the invention may be carried out in other ways without departing from the spirit of my invention and, therefore, I claim broadly the right to employ all equivalent instrumentalities coming within the scope of the appended claim, and by means of which, objects of my invention are attained and new results ac- Cdmplished, as it is obvious that the particular embodiments herein shown are only some of the many that can be employedto attain these objects and accomplish these results.

I claim:

' In a protective-system for the electric circuit of an aircraft, comprising a housing, a pair of outer contacts, an auxiliary electric heater in series with said contacts, a fusible element made of relatively high melting point metal in contact with said auxiliary heater, said fusible element having the physical characteristic of remaining 15 2,780,696

solid at all temperatures below its melting point and joined in series therewith, a heat sink at the junction of the auxiliary heater and said fusible element, said heat References Cited in the tile of this patent UNITED STATES PATENTS 1,626,105 Sundt Apr. 26, 1927 1,757,753 Thiery et a1. May 6, 1930 2,225,287 Sundt Dec. 17, 1940 Lear Feb. 5, 1957 

